As is well known, semiconductor processing is most commonly employed for the fabrication of integrated circuits, which entails particularly stringent quality demands, but such processing is also employed in a variety of other fields. For example, semiconductor processing techniques are often employed in the fabrication of flat panel displays using a wide variety of technologies and in the fabrication of microelectromechanical systems.
A variety of methods are used in the semiconductor manufacturing industry to deposit materials onto surfaces. For example, one of the most widely used methods is chemical vapor deposition, in which atoms or molecules contained in a precursor vapor deposit on a surface and build up to form a film. In some contexts, it is desirable to deposit selectively within semiconductor windows exposed among fields of different materials, such as field isolation oxide. For example, heterojunction bipolar transistors are often fabricated using selective deposition techniques that deposit epitaxial (single-crystal) semiconductor films only on active areas. Other transistor designs benefit from elevated source/drain structures, which provide additional silicon that can be consumed by the source/drain contact process without altering shallow junction device performance. Selective epitaxy on source/drain regions advantageously reduces the need for subsequent patterning and etch steps
The film growth is based on surface reactions that take place on the surface of the substrate to form a solid-state layer of atoms or molecules, because the precursors and the temperature of the substrate are chosen such that the alternately injected vapor phase precursor's molecules react only on the substrate's surface layer. The precursors may also be injected in sufficiently high doses for the surface to be fully saturated during each injection cycle.
Multiple precursor pulses are needed to form various layers onto the substrate, and the pulses may have to be kept separated from each other to prevent uncontrolled growth of the film, contamination of the reactor chamber, undesired reactions between precursors. After each pulse, the gaseous reaction products of the thin-film growth process as well as the excess reactants in vapor phase have to be removed from the reactor chamber. This can be achieved either by pumping down the reactor chamber or by purging the reactor chamber with a gas flow between successive pulses. For example, in the latter method, a column of an inactive or inert gas is introduced in the conduits between the precursor pulses. Regularly, the purging gas is also used as a carrier gas during precursor pulses.
In existing reactor chambers, process gases are commonly premixed together with some amount of a sweep purge carrier gas and stabilized to a vent before being combined with the main carrier gas and injected into the reactor chamber. For example, the pressure in a deposition line depends on the carrier gas flow, typically in the range of 10-100 standard liters per minute (slm), the resistance of the multiport injector causing a pressure drop, and the pressure downstream from the multiport injector inside the reactor chamber. When the pressure in the deposition line becomes steady the entire gas flow passes the multiport injector. However, if the deposition line pressure increases, the amount of gas entering the deposition line through one or more metering mass flow controllers exceeds the amount of mixed gas exiting the gas panel through the multiport injector. Conversely, if the deposition line pressure decreases, the amount of mixed gas exiting the gas panel through the multiport injector exceeds the amount of gas entering the deposition line through the metering mass flow controllers. This behavior can become very dynamic, in particular if:                large amounts of process gas were switched at the same time;        the steps are very short (and potentially repeated);        the amount of process gas relative to the carrier gas is high (small carrier flow);        the carrier gas flow is dramatically changed; and        
the reactor pressure downstream the multiport injector is changed (periodically).
By restricting the outlet of the vent with an adjustable needle valve the pressure in the vent line can be matched with the pressure in the deposition line. But this works only for one particular total flow, e.g. the highest process gas flows in the recipe, such as when a combination of gases comprising H2, HCl, DCS/SiH4, GeH4, SiH3CH3, and/or B2H6 are flowing all together into the reactor chamber at the same time. To ensure the absence of Ge or B pressure spikes, it is common to introduce the gases in a certain order and to ramp some gases, such as GeH4 and B2H6, slowly to an appropriate set point to avoid overdoping.
Using this method, the partial pressure of each individual gas changes when one or more additional gases are added or removed, even when the various mass flow controller set points for given gases are kept constant. To determine the right vent pressure, the process typically has to be run first and the pressure sensors monitored. Then the needle valve needs to be adjusted until the pressures are matched for the desired deposition step. This process is time consuming and can normally be done properly only for one production recipe. Changing the total gas flow also changes the gas velocity, the precursor partial pressures of the individual process gasses, and the composition over time all of which can affect the uniformity and the total thickness of the desired layer. This can be problematic for processes in the mass flow limited regime or in the transition region especially when using very low main carrier flows.